Non-linear optical device sensitive to green laser

ABSTRACT

A non-linear optical device comprising a polymer configured to be photorefractive upon irradiation by a green laser. The polymer comprises a repeating unit including a moiety selected from the group consisting of the structures (M-1), (M-2) and (M-3), as defined herein.

This application claims priority to U.S. Provisional Application No. 60/833,430, entitled “NON-LINEAR OPTICAL DEVICE SENSITIVE TO GREEN LASER,” filed on Jul. 25, 2006, the contents of which are incorporated herein by reference in their entirety.

This application is related to and incorporates by reference in its entirety U.S. patent application Ser. No. 11/781,508 to Yamamoto, entitled “NON-LINEAR OPTICAL DEVICE WITH LONG GRATING PERSISTENCY,” which is now U.S. Pat. No. 7,736,548, filed concurrently herewith on Jul. 23, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a non-linear optical device comprising a polymer configured to be photorefractive upon irradiation by a green laser. More particularly, the polymer comprises a repeating unit including a moiety selected from the group consisting of carbazole moiety, tetraphenyl diaminobiphenyl moiety and triphenylamine moiety. In addition, this invention relates to a method for modulating light using the polymer that is irradiated by a green laser.

2. Description of the Related Art

Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by laser beam irradiation. The change of the refractive index typically involves: (1) charge generation by laser irradiation, (2) charge transport, resulting in the separation of positive and negative charges, (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization and (5) a refractive index change induced by the non-uniform electric field. Good photorefractive properties are typically observed in materials that combine good charge generation, charge transport or photoconductivity and electro-optical activity. Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition. Particularly, long lasting grating behavior can contribute significantly for high-density optical data storage or holographic display applications.

Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO₃. In these materials, the mechanism of a refractive index modulation by the internal space-charge field is based on a linear electro-optical effect.

In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable depending on the application include sufficiently long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.

In recent years, efforts have been made to optimize the properties of organic, and particularly polymeric, photorefractive materials. Various studies have been done to examine the selection and combination of the components that give rise to each of these features. The photoconductive capability is frequently provided by incorporating materials containing carbazole groups. Phenyl amine groups can also be used for the charge transport part of the material.

Non-linear optical ability is generally provided by including chromophore compounds, such as an azo-type dye that can absorb photon radiation. The chromophore may also provide adequate charge generation. Alternatively, a material known as a sensitizer may be added to provide or boost the mobile charge for photorefractivity to occur.

The photorefractive composition may be made by mixing molecular components that provide desirable individual properties into a host polymer matrix. However, most of previously prepared compositions failed to show good photorefractivity performances, (e.g., high diffraction efficiency, fast response time and long-term stability). Efforts have been made, therefore, to provide compositions which show high diffraction efficiency, fast response time and long stability.

U.S. Pat. Nos. 6,653,421 B1 and 6,610,809 B1 disclose (meth)acrylate-based polymers and copolymer based materials which showed high diffraction efficiency, fast response time, and long-term phase stability. The materials show fast response times of less than 30 msec and diffraction efficiency of higher than 50%, along with no phase separation for at least two or three months.

Several green laser sensitive-organic based materials and holographic medium have been developed. U.S. Pat. Nos. 3,658,526, 4,942,112, 4,959,284, 4,994,347, 4,588,664, 4,696,876 and 4,970,129 disclose green laser sensitive photopolymerizable materials which can be used for holographic data storages and memory applications. However, these types of materials are write-once type materials and cannot be rewritten after a single use.

Also, US 2004/0043301 discloses a data storage medium, comprising a recording layer containing molecules having charge transport characteristics, molecules having nonlinear optical characteristics, and optical functional molecules whose stereostructure is changed depending on a light irradiation, and a pair of transparent ohmic electrodes sandwiching the recording layer. The conductivity of the data storage medium is lowered by the light irradiation. However, the diffraction efficiency immediately after the recording was found to be 1.0%. It is not really effective for actual device application.

None of the materials described above achieves the desired combination of high diffraction efficiency with a fast response time and long-term stability, along with rewritable properties. Furthermore, none of the materials described above showed sensitivities for green pulse laser and optimum combination of high diffraction efficiency. Usually, continuous wave (CW) laser system can be affected by vibration during measurement and laser operations, but pulse laser is vibration free and widely used. Green pulse laser system availability can be greatly advantageous and useful from industrial application purpose and image storage purpose.

SUMMARY OF THE INVENTION

It is one object of several embodiments of the present invention to provide a photorefractive composition which exhibits fast response time and high diffraction efficiency, along with long diffractive grating lasting time and phase stability.

An embodiment is directed to a non-linear optical device comprising a polymer configured to be photorefractive upon irradiation by a green laser, wherein the polymer comprises a repeating unit that includes a moiety selected from the group consisting of the following formulas:

wherein each Q in (M-1), (M-2), and (M-3) independently represents an unsubstituted or substituted alkylene group; and wherein Ra₁-Ra₈, Rb₁-Rb₂₇ and Rc₁-Rc₁₄ in (M-1), (M-2), and (M-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein the alkyl may be branched or linear.

Another embodiment provides a method for modulating light, comprising the steps of: providing a photorefractive composition comprising a polymer, wherein the polymer comprises a repeating unit that includes a moiety selected from the group consisting of the following structures:

wherein each Q in (M-1), (M-2), and (M-3) independently represents an unsubstituted or substituted alkylene group; wherein Ra₁-Ra₈, Rb₁-Rb₂₇ and Rc₁-Rc₁₄ in (M-1), (M-2), and (M-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, wherein the alkyl may be branched or linear, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl; and irradiating the photorefractive composition with a green laser to thereby modulate a photorefractive property of the polymer.

Embodiments of the invention have great utility in a variety of optical applications, including holographic storage, optical correlation, phase conjugation, non-destructive evaluation and imaging. Embodiments of non-linear optical devices include hologram medium device, hologram memory device, optical limiter, image correction device and correlator device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction illustrating a hologram recording system with a photorefractive polymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Non-Linear Optical Devices

An embodiment provides a non-linear optical device comprising a polymer configured to be photorefractive upon irradiation by a green laser, wherein the polymer comprises a repeating unit that includes a moiety selected from the group consisting of a carbazole moiety (represented by the structure (M-1)), a tetraphenyl diaminobiphenyl moiety (represented by the structure (M-2)), and a triphenylamine moiety (represented by the structure (M-3)). These moieties are represented by the following formulas:

wherein each Q in (M-1), (M-2), and (M-3) independently represents an unsubstituted or substituted alkylene group, and Ra₁-Ra₈, Rb₁-Rb₂₇ and Rc₁-Rc₁₄ in (M-1), (M-2), and (M-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein the alkyl may be branched or linear. Preferably each Q in (M-1), (M-2), and (M-3) is independently an alkylene group represented by (CH₂)_(p) where p is in the range of about 2 and about 6.

In another embodiment, the polymer that is configured to be photorefractive upon irradiation by a green laser may comprise a second repeating unit that includes a second moiety represented by formula (M-4):

wherein Q in (M-4) independently represents an unsubstituted or substituted alkylene group, R₁ in (M-4) is selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, wherein the alkyl may be branched or linear, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, G in (M-4) is a p-conjugated group and Eacpt in (M-4) is an electron acceptor group. Preferably Q in (M-4) is an alkylene group represented by (CH₂)_(p) where p is between about 2 and about 6. The Q in formula (M-4) may be the same or different from the Q in formulas (M-1), (M-2) and (M-3).

In some embodiments, “G=” in formula (M-4) may be represented by a structure selected from the group consisting of the following formulas:

wherein Rd₁-Rd₄ and R₂ in (G-1) and (G-2) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein the alkyl may be branched or linear.

In some embodiments, Eacpt in formula (M-4) may be represented by a structure selected from the group consisting of the following formulas:

wherein R₅, R₆, R₇ and R₈ in (E-2), (E-3), and (E-5) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein the alkyl may be branched or linear.

In some embodiments, the polymer in the non-linear optical device may further comprise a plasticizer and/or a sensitizer. In some embodiments, the polymer is configured to be photorefractive upon irradiation by a green laser at a wavelength of 532 nm. In some embodiments, the polymer has an over 30% transmittance at a film thickness of 100 μm when irradiated by the green laser. Some embodiments provide a non-linear optical device comprising the polymer that is configured to be photorefractive upon irradiation by a green laser that is a continuous wave laser or a pulse laser.

Many polymer backbones, including but not limited to, polyurethane, epoxy polymers, polystyrene, polyether, polyester, polyamide, polyimide, polysiloxane, and polyacrylate, with the appropriate side chains attached can be used to make the polymers described herein. Some embodiments contain backbone units based on acrylates or styrene, and one of many preferred backbone units are formed from acrylate-based monomers, and another is formed from methacrylate monomers. It is believed that the first polymeric materials to include photoconductive functionality in the polymer itself were the polyvinyl carbazole materials developed at the University of Arizona. However, these polyvinyl carbazole polymers tend to become viscous and sticky when subjected to the heat-processing methods typically used to form the polymer into films or other shapes for use in photorefractive devices.

The (meth)acrylate-based and the acrylate-based polymers used in many embodiments of the invention have much better thermal and mechanical properties. In other words, they provide better workability during processing by injection-molding or extrusion, especially when the polymers are prepared by radical polymerization.

Some embodiments provide a non-linear optical device comprising a polymer configured to be photorefractive upon irradiation by a green laser, wherein the polymer comprises a first repeating unit selected from the group consisting of the following formulas:

wherein each Q in (U-1), (U-2), and (U-3) independently represents an unsubstituted or substituted alkylene group, and wherein Ra₁-Ra₈, Rb₁-Rb₂₇ and Rc₁-Rc₁₄ in (U-1), (U-2), and (U-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl. The alkyl group can be either branched or linear alkyl. In some embodiments, each Q in (U-1), (U-2), and (U-3) is an alkylene group represented by (CH₂)_(p) where p is between about 2 and about 6. In some embodiments, each Q in (U-1), (U-2), and (U-3) is independently selected from the group consisting of ethylene, propylene, butylene, pentylene, hexylene, and heptylene.

Non-limiting examples of monomers that may be copolymerized to form a charge transport component include those comprising a phenyl amine derivative group, such as carbazolylpropyl (meth)acrylate monomer, 4-(N,N-diphenylamino)-phenylpropyl (meth)acrylate, N-[(meth)acroyloxypropylphenyl]-N,N′,N′-triphenyl-(1,1′-biphenyl)-4,4′-diamine; N-[(meth)acroyloxypropylphenyl]-N′-phenyl-N,N′-di(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and N-[(meth)acroyloxypropylphenyl]-N′-phenyl-N,N′-di(4-buthoxyphenyl)-(1,1′-biphenyl)-4,4′-diamine. These monomers can be used by themselves or as a mixture of two or more monomers.

In some embodiments, the polymer used in the non-linear optical device further comprises a second repeating unit represented by the following formula:

wherein Q in (U-4) independently represents an unsubstituted or substituted alkylene group, R₁ in (U-4) is selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, wherein the alkyl may be branched or linear, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, G (in U-4) is a p-conjugated group and Eacpt in (U-4) is an electron acceptor group. Preferably Q in (U-4) is an alkylene group represented by (CH₂)_(p) where p is between about 2 and about 6. The alkyl group can be either branched or linear alkyl. In some embodiments, R₁ in (U-4) is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyl. In some embodiments, Q in (U-4) is selected from the group consisting of ethylene, propylene, butylene, pentylene, hexylene, and heptylene.

The term “p-conjugated group” refers to a molecular fragment that contains p-conjugated bonds. The p-conjugated bonds refer to covalent bonds between atoms that have s bonds and p bonds formed between two atoms by overlapping of atomic orbits (s+p hybrid atomic orbits for s bonds and p atomic orbits for p bonds). “G=” in formula (U-4) may be represented by a structure selected from the group consisting of the following formulas:

wherein Rd₁-Rd₄ and R₂ in (G-3) and (G-4) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein the alkyl may be branched or linear. In an embodiment, Rd₁-Rd₄ in (G-3) and (G-4) are hydrogen.

The term “electron acceptor group” refers to a group of atoms with a high electron affinity that can be bonded to a p-conjugated group. Exemplary electron acceptors, in order of increasing strength, are: C(O)NR²<C(O)NHR<C(O)NH₂<C(O)OR<C(O)OH<C(O)R<C(O)H<CN<S(O)₂R<NO₂, wherein each R in these electron acceptors may independently be, for example, hydrogen, C₁-C₁₀ alkyl, and C₄-C₁₀ aryl, wherein the alkyl may be branched or linear. Eacpt in formula (U-4) may be represented by a structure selected from the group consisting of the following formulas:

wherein R₅, R₆, R₇ and R₈ in (E-7), (E-8), and (E-10) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein alkyl may be branched or linear.

The polymers described herein may be prepared in various ways, e.g., by polymerization of the corresponding monomers or precursors thereof. Polymerization may be carried out by methods known to a skilled artisan, as informed by the guidance provided herein. In some embodiments, radical polymerization using an azo-type initiator, such as AIBN (azoisobutyl nitrile), may be carried out. The radical polymerization technique makes it possible to prepare random or block copolymers comprising both charge transport and non-linear optical groups. Further, by following the techniques described herein, it is possible to prepare such materials with exceptionally good properties, such as photoconductivity, response time and diffraction efficiency. In an embodiment of a radical polymerization method, the polymerization catalysis is generally used in an amount of from 0.01 to 5 mole % or from 0.1 to 1 mole % per mole of the total polymerizable monomers. In some embodiments, radical polymerization can be carried out under inert gas (e.g., nitrogen, argon, or helium) and/or in the presence of a solvent (eg., ethyl acetate, tetrahydrofuran, butyl acetate, toluene or xylene). Polymerization may be carried out under a pressure from 1 to 50 Kgf/cm² or from 1 to 5 Kgf/cm². In some embodiments, the concentration of total polymerizable monomer in a solvent may be about 0.1% to about 80% by weight, preferably about 0.5% to about 60%, more preferably about 1% to about 40%, and even more preferably about 1% to about 30% by weight. The polymerization may be carried out at a temperature of about 50° C. to about 100° C., and may be allowed to continue for about 1 to about 100 hours, depending on the desired final molecular weight, polymerization temperature, and taking into account the polymerization rate.

Non-limiting examples of monomers that may be polymerized to form a non-linear optical component include those that comprise a chromophore group, such as N-ethyl, N-4-dicyanomethylidenyl acrylate, N-ethyl, N-4-dicyanomethylidenyl-3,4,5,6,10-pentahydronaphtylpentyl acrylate and any combination thereof. To prepare the non-linear optical containing copolymer, monomers that have side-chain groups possessing non-linear-optical ability may be used. Non-limiting examples of such monomers include:

wherein each Q in the monomers above independently represent an unsubstituted or substituted alkylene group, each R₀ in the monomers above is independently hydrogen or methyl, and each R in the monomers above is independently C₁-C₁₀ alkyl. In some embodiments, Q in the monomers above may be an alkylene group represented by (CH₂)_(p) where p is in the range of about 2 to about 6. The alkylene group may contain one or more hetero atoms (e.g., O or S). In some embodiments, each R in the monomers above may be independently selected from the group consisting methyl, ethyl and propyl. Each R₀ in the monomers above may be independently H or CH₃.

A new polymerization method involving the use of a precursor monomer with a functional group for non-linear optical ability can be used to prepare the copolymers. In some embodiments, the precursor may be represented by the following formula:

wherein R₀ in (P-1) is hydrogen or methyl and V in (P-1) is selected from (V-1) or (V-2):

wherein each Q in (V-1) and (V-2) independently represents an unsubstituted or substituted alkylene group, Rd₁-Rd₄ in (V-1) and (V-2) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl, and R₁ in (V-1) and (V-2) is C₁-C₁₀ alkyl (branched or linear). In some embodiments, Q in (V-1) and (V-2) may independently be an alkylene group represented by (CH₂)_(p) where p is in the range of about 2 to about 6. The alkylene group may contain one or more hetero atoms (e.g., O or S). In some embodiments, R₁ in (V-1) and (V-2) is independently selected from a group consisting of methyl, ethyl, propyl, butyl, pentyl and hexyl. In an embodiment, Rd₁-Rd₄ in (V-1) and (V-2) are hydrogen.

This new polymerization method works under the same initial operating conditions as for the conventional radical polymerization described above, and it also follows the same procedure to form the precursor polymer. After the precursor copolymer has been formed, it can be converted into the corresponding copolymer having non-linear optical groups and capabilities by a condensation reaction. In some embodiments, the condensation reagent may be selected from the group consisting of:

wherein R₅, R₆, R₇ and R₈ of the condensation reagents above are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl and C₄-C₁₀ aryl. The alkyl group may be either branched or linear.

The condensation reaction can be done in the presence of a pyridine derivative catalyst at room temperature for about 1-100 hrs. In some embodiments, a solvent, such as butyl acetate, chloroform, dichloromethylene, toluene or xylene, can also be used. In some embodiments, the reaction may be carried out without the catalyst at a solvent reflux temperature of 30° C. or above for about 1 to 100 hours.

In some embodiments, the non-linear optical device may comprise a copolymer that provides photoconductive (charge transport) ability and non-linear optical ability. The optical device may also include other components as desired, such as sensitizer and/or plasticizer components. Some embodiments provide a non-linear optical device that comprises a copolymer. The copolymer may comprise a first repeating unit that includes a first moiety with charge transport ability, a second repeating unit including a second moiety with non-linear-optical ability, and a third repeating unit that include a third moiety with plasticizing ability.

Those having ordinary skill in the art, using routine experimentation guided by the disclosure herein, can vary the components of the polymer to make a photorefractive polymer that is sensitive to a green laser. For example, specific examples of polymers sensitive to a green laser are described in the examples below. Using such polymers as a starting point, those skilled in the art can use routine experimentation to identify other polymers that are also sensitive to a green laser. For example, the molecular weight of the polymer may be varied over a broad range. In addition, the ratio of differing types of repeating units may be varied to create a polymer that is sensitive to a green laser. For example the ratio of a first repeating unit that includes a first moiety with charge transport ability to a second repeating unit including a second moiety with non-linear-optical ability may be varied over a broad range. Additionally, a third repeating unit that includes a third moiety with plasticizing ability may further be provided in a variety of ratios to the first repeating unit and/or the second repeating unit.

In addition, the ratio of different types of monomers used in forming the copolymer may be varied over a broad range. Some embodiments may provide an optical device with the first repeating unit (e.g., the repeating unit with charge transport ability) to the second repeating unit (e.g., the repeating unit with non-linear optical ability) weight ratio of about 100:1 to about 0.5:1, preferably about 10:1 to about 1:1. When the weight ratio is less than 0.5:1, the charge transport ability of copolymer may be relatively weak, and the response time may be undesirably slow to give good photorefractivity. However, even in this case, the addition of already described low molecular weight components having non-linear-optical ability can enhance photorefractivity. On the other hand, if this weight ratio is larger than about 100:1, the non-linear-optical ability of the copolymer may be relatively weak, and the diffraction efficiency may be too low to give good photorefractivity. In this case, the addition of already described low molecular weight components having charge transport ability can also enhance photorefractivity.

In some embodiments, the molecular weight and the glass transition temperature, Tg, of the copolymer are selected to provide desirable physical properties. In some embodiments, it is valuable and desirable, although not essential, that the polymer is capable of being formed into films, coatings and shaped bodies of various kinds by standard polymer processing techniques (e.g., solvent coating, injection molding or extrusion).

In some embodiments, the polymer has a weight average molecular weight, Mw, of from about 3,000 to about 500,000, preferably from about 5,000 to about 100,000. The term “weight average molecular weight” as used herein means the value determined by the GPC (gel permeation chromatography) method using polystyrene standards, as is well known in the art.

As another component of the non-linear optical device, the chromophore or group that provides the non-linear optical functionality may be any group known in the art that can provide such capability. In some embodiments, these non-linear optical components may be incorporated into the polymer matrix by attaching to the polymer as a side chain and/or as additive components.

Preferably, the chromophore or group that provides the non-linear optical functionality is represented by (M-4):

wherein Q in (M-4) independently represents an unsubstituted or substituted alkylene group, R₁ in (M-4) is selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, wherein the alkyl may be branched or linear, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, G in (M-4) is a p-conjugated group, and Eacpt is an electron acceptor group. In some embodiments, Q in (M-4) may be an alkylene group represented by (CH₂)_(p) where p is in the range of about 2 to about 6. In some embodiments Q in (M-4) may be selected from the group consisting of ethylene, propylene, butylene, pentylene, hexylene, and heptylene. In some embodiments, R₁ in (M-4) may be selected from the group consisting of methyl, ethyl, propyl, butyl, pentyl and hexyl.

Functional groups disclosed in U.S. Pat. No. 6,267,913 may be useful as an electron acceptor group in some embodiments:

wherein each R in the functional groups above is independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl and C₄-C₁₀ aryl, wherein the alkyl may be branched or linear. The symbol “‡” in a chemical structure throughout this application specifies an atom of attachment to another chemical group and indicates that the structure is missing a hydrogen that would normally be implied by the structure in the absence of the “‡.”

In some embodiments, “G=” in formula (M-4) is a non-linear optical functionality, and may be represented by a structure selected from the group consisting of the following formulas:

wherein Rd₁-Rd₄ and R₂ in (G-1) and (G-2) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein the alkyl may be branched or linear. In some embodiments, derivatives of the following structures may be useful as a non-linear optical functionality:

wherein each R in the structures above is independently a group selected from the group consisting of hydrogen, C₁-C₁₀ alkyl and C₄-C₁₀ aryl, wherein the alkyl may be branched or linear.

Other chromophores that possess non-linear optical properties in a polymer matrix are described in U.S. Pat. No. 5,064,264 (incorporated herein by reference) may also be used in some embodiments. Additional suitable materials known in the art may also be used, and are well described in the literature, such as D. S. Chemla & J. Zyss, “Nonlinear Optical Properties of Organic Molecules and Crystals” (Academic Press, 1987). U.S. Pat. No. 6,090,332 describes fused ring bridge and ring locked chromophores that can form thermally stable photorefractive compositions, which may be useful as well. In some embodiments, chromophore additives having the following chemical structures can be used.

wherein each R in the chromophore additives above is independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl and C₄-C₁₀ aryl, wherein the alkyl may be branched or linear.

The chosen compound(s) is sometimes mixed in the copolymer in a concentration of about 1% to about 50% by weight. The polymer can be mixed with a component that possesses plasticizer properties, and any commercial plasticizer compound can be used, such as phthalate derivatives or low molecular weight hole transfer compounds (e.g., N-alkyl carbazole or triphenylamine derivatives or acetyl carbazole or triphenylamine derivatives).

Non-limiting examples of plasticizer compounds include ethyl carbazole; 4-(N,N-diphenylamino)-phenylpropyl acetate; 4-(N,N-diphenylamino)-phenylmethyloxy acetate; N-(acetoxypropylphenyl)-N,N′,N′-triphenyl-(1,1′-biphenyl)-4,4′-diamine; N-(acetoxypropylphenyl)-N′-phenyl-N,N′-di(4-methylphenyl)-(1,1′-biphenyl)-4,4′ -diamine; N-(acetoxypropylphenyl)-N′-phenyl-N,N′-di(4-butoxyphenyl)-(1,1′-biphenyl)-4,4′-diamine and combination thereof. These compounds can be used singly or in mixtures of two or more plasticizers. Furthermore, un-polymerized monomers can be plasticizers and/or low molecular weight hole transfer compounds, for example 4-(N,N-diphenylamino)-phenylpropyl (meth)acrylate; N-[(meth)acroyloxypropylphenyl]-N,N′,N′-triphenyl-(1,1′-biphenyl)-4,4′-diamine; N-[(meth)acroyloxypropylphenyl]-N′-phenyl-N,N′-di(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine; and N-[(meth)acroyloxypropylphenyl]-N′-phenyl-N,N′-di(4-buthoxyphenyl)-(1,1′-biphenyl)-4,4′-diamine. These monomers can also be used singly or in mixtures of two or more monomers.

In some embodiments, plasticizers like N-alkyl carbazole or triphenylamine derivatives containing electron acceptor group can stabilize the photorefractive composition, because the plasticizers contain both N-alkyl carbazole or triphenylamine moiety and non-linear optical moiety in one compound. The plasticizer may be selected from the following formulas:

wherein Ra₁, Rb₁-Rb₄ and Rc₁-Rc₃ in (P-1), (P-2), and (P-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, wherein the alkyl may be branched or linear, and C₄-C₁₀ aryl, z is 0 or 1, and Eacpt in (P-1), (P-2), and (P-3) is independently an electron acceptor group and represented by a structure selected from the group consisting of the following formulas:

wherein R₅, R₆, R₇ and R₈ in (E-12), (E-13), and (E-15) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl, wherein alkyl may be branched or linear.

Embodiments of this invention provide polymers having comparatively low glass transition temperature (T_(g)) when compared with similar polymers prepared by prior art methods. By selecting copolymers of moderate Tg and by using methods that tend to depress the average Tg, it is possible to limit the amount of plasticizer required for the composition and reduce or eliminate the inclusion of plasticizers. In some embodiments, the amount of plasticizer may be reduced so that it is about 1 to about 30% of the total composition, including about 1 to about 25% of the total composition, or about 1 to about 20% of the total composition.

In some embodiments, other components may be added to the polymer matrix to provide or improve the desired physical properties for a non-linear optical device. In an embodiment, a photosensitizer may be added to serve as a charge generator for good photorefractive capability. A wide variety of such photosensitizers is known in the art. In some embodiments, useful photosensitizers may be 2,4,7-trinitro-9-fluorenone dicyanomalonate (TNFDM), dinitro-fluorenone, mononitro-fluorenone and C60, and the amount of photosensitizer included is usually about 0.5 to about 3% by weight, based on the total weight.

Embodiments of this invention show very good phase stabilities and give essentially no haziness even after several months. Without phase separation, good photorefractive properties remain.

FIG. 1 is a schematic depiction illustrating a hologram recording system with a photorefractive polymer. Information may be recorded into the hologram medium, and the recorded information may be read out simultaneously. A laser source 11 may be used as to write information onto a recording medium 12. The recording medium 12 comprises the photorefractive polymer described herein and is positioned over a support material 13.

Laser beam irradiation of object beam 14 and reference beam 16 into the recording medium 12 causes interference grating, which generates internal electric fields and a refractive index change. Multiple recordings are possible in the photorefractive polymer of the recording medium 12 by changing the angle of the incident beam. The object beam 14 has a transmitted portion 15 of the beam and a refracted portion 17 of the beam.

An image display device 19 is set up parallel to the X-Y plane of the recording medium 12. Various types of image display devices may be employed. Some non-limiting examples of image display devices include a liquid crystal device, a Pockels Readout Optical Modulator, a Multichannel Spatial Modulator, a CCD liquid crystal device, an AO or EO modulation device, or an opto-magnetic device. On the other side of the recording medium 12, a read-out device 18 is also set up parallel to the X-Y plane of the recording medium 12. Suitable read-out devices include any kind of opto-electro converting devices, such as CCD, photo diode, photoreceptor, or photo multiplier tube.

In order to read out recorded information, the object beam 14 is shut out and only the reference beam 16, which is used for recording, is irradiated. A reconstructed image may be restored, and the reading device 18 is installed in the same direction as the transmitted portion 15 of the object beam and away from the reference beam 16. However, the position of the reading device 18 is not restricted to the positioning shown in FIG. 1. Recorded information in the photorefractive polymer can be erased completely by whole surface light irradiation, or partially erased by laser beam irradiation.

The method can build the diffraction grating on the recording medium. This hologram device can be used not only for optical memory devices but also other applications, such as a hologram interferometer.

For embodiments of the non-linear optical device, the thickness of a photorefractive polymer layer is from about 10 μm to about 200 μm. In some embodiments, the thickness range is between about 30 μm and about 150 μm. A suitable range of transmittance is between about 10% and about 99%, including between about 30% and about 99%. In some embodiments, the transmittance is between about 10% and about 99.99%, including between about 30% and 99.9%. In an embodiment, the polymer has an over 30% transmittance at a thickness of 100 μm when irradiated by a green laser at a wavelength of 532 nm.

Linear transmittance may be performed to determine the absorption coefficient of the photorefractive device. For measurements, 532 nm laser beam is positioned so that the incident beam is perpendicular to the non-linear optical device, the incident and exit beam intensities are monitored and the linear transmittance of the sample is given by: T=I _(Transmitted) /I _(incident)

A green laser is a laser which emits light at wavelengths between 500 nm and 570 nm. In an embodiment, a green laser light source at a wavelength of 532 nm laser can be used.

Furthermore, some embodiments show sensitivity to both green continuous wave laser and green pulse laser. However, continuous wave laser system can be affected by vibration during measurement and laser operations, while pulse laser is vibration free and more widely used. Thus, pulse laser system availability can be greatly advantageous and useful for industrial application purpose and image storage purpose.

One of the advantageous features of preferred embodiments is the fast response time. The response time is the time needed to build up the diffraction grating in the photorefractive material when exposed to a laser writing beam. A faster response time means faster grating build-up, which enables the photorefractive composition or the polymer to be used for wider applications, such as real-time hologram applications. The response time of a material may be measured by transient four-wave mixing (TFWM) experiments, as detailed in the Examples section below. The data may then be fitted with the following bi-exponential function: ?(t)=sin²{?₀(1−a ₁ e ^(−t/J1−) a ₂ e ^(−t/J2))²} wherein a₁+a₂=1, ? (t) is the diffraction efficiency at time t, ?₀ is the steady-state diffraction efficiency, and J₁ and J₂ are the grating build-up times. The smaller number of J₁ and J₂ is defined as the response time.

Furthermore, the fast response time can be achieved without resorting to a very high electric field (i.e., biased voltage), such as a field in excess of about 100 V/μm. Embodiments of this invention can achieve a fast response time at a biased voltage no higher than about 100 V/μm. Some embodiments can even achieve a fast response time at less than about 90 V/μm.

Another advantageous feature of preferred embodiments is the high diffraction efficiency, ?. Diffraction efficiency is defined as the ratio of the intensity of a diffracted beam to the intensity of an incident probe beam, and is determined by measuring the intensities of the respective beams. In general, for a given photorefractive composition, a higher diffraction efficiency can be achieved by increasing the applied biased voltage. Preferred embodiments provide good diffraction efficiencies, such as at least about 5%, and some embodiments may achieve at least about 10%.

Method for Modulating Light

Another embodiment is directed to a method for modulating light, comprising the steps of providing a photorefractive composition comprising a polymer as described herein, e.g., a polymer that comprises a repeating unit that includes a moiety selected from the group consisting of the (M-1), (M-2) and (M-3) as described above; and irradiating the photorefractive composition with a green laser to thereby modulate a photorefractive property of the polymer.

In some embodiments, the method utilizes a polymer that comprises a second repeating unit that includes a second moiety represented by formula (M-4) as described above.

Another embodiment is directed to a method for modulating light, comprising the steps of providing a photorefractive composition comprising a polymer as described herein, e.g., a polymer that comprises a repeating unit selected from the group consisting of (U-1), (U-2) and (U-3) as described above, and irradiating the photorefractive composition with a green laser to thereby modulate a photorefractive property of the polymer.

In some embodiments, the method utilizes a polymer (e.g., copolymer), that comprises a second repeating unit represented formula (U-4) as described above.

In some embodiments, the photorefractive composition used in the methods described above may further comprise a plasticizer and/or a sensitizer. The photorefractive composition may have an over 30% transmittance at a thickness of 100 μm. Some embodiments provide a method for modulating light, comprising the steps of providing a photorefractive composition described above and irradiating the photorefractive composition with a green laser that is configured to irradiate the composition at a wavelength of about 532 nm. In some embodiments, the green laser may be a continuous wave laser or a pulse laser.

The embodiments will now be further described by the following examples, which are intended to be illustrative of the embodiments, and do not limit the scope or underlying principles in any way.

EXAMPLES

(a) Monomers Containing Charge Transport Groups

TPD acrylate monomer: TPD acrylate type charge transport monomers (N-[acroyloxypropylphenyl]-N,N′,N′-triphenyl-(1,1′-biphenyl)-4,4′-diamine) (TPD acrylate) was purchased from Fuji Chemical, Japan, and it has the following structure:

(b) Monomers Containing Non-linear-optical Groups

The non-linear-optical precursor monomer 5-[N-ethyl-N-4-formylphenyl]amino-pentyl acrylate was synthesized according to the following synthesis scheme:

Step I:

Triethylamine (4.2 mL, 30 mmol) and N-ethylaniline (4 mL, 30 mmol) were added to bromopentyl acetate (5 mL, 30 mmol) and toluene (25 mL) at room temperature, and then heated at 120° C. overnight. After cooling down, the reaction mixture was rotary-evaporated. The residue was purified by silica gel chromatography (developing solvent: hexane/acetone=9/1). An oily amine compound was obtained (yield: 6.0 g, 80%).

Step II:

Anhydrous DMF (6 mL, 77.5 mmol) was cooled in an ice-bath. Then POCl₃ (2.3 mL, 24.5 mmol) was added dropwise to anhydrous DMF in a 25 mL flask, and the mixture was allowed to come to room temperature. The amine compound (5.8 g, 23.3 mmol) was diluted with dichloroethane (6 mL) and then added to the POCl₃ mixture through a rubber septum using a syringe. After stirring for 30 min., this reaction mixture was heated to 90° C. and the reaction was allowed to proceed overnight under an argon atmosphere.

After the reaction overnight, the reaction mixture was cooled, and poured into brine water and extracted by ether. The ether layer was washed with potassium carbonate solution and dried over anhydrous magnesium sulfate. After removing the magnesium sulfate, the solvent was removed and the residue was purified by silica gel chromatography (developing solvent: hexane/ethyl acetate=3/1). An aldehyde compound was obtained (yield: 4.2 g, 65%).

Step III:

The aldehyde compound (3.92 g, 14.1 mmol) was dissolved in methanol (20 mL). Potassium carbonate (400 mg) and water (1 mL) were added to the aldehyde compound/methanol mixture at room temperature and let stirred overnight. After stirring overnight, the solution was poured into brine water and extracted by ether. The ether layer was dried over anhydrous magnesium sulfate. After removing the magnesium sulfate, the solvent was removed and the residue was purified by silica gel chromatography (developing solvent: hexane/acetone=1/1). An aldehyde alcohol compound was obtained (yield: 3.2 g, 96%).

Step IV:

The aldehyde alcohol (5.8 g, 24.7 mmol) was dissolved with anhydrous THF (60 mL). Into this mixture, triethylamine (3.8 mL, 27.1 mmol) was added and the solution was cooled by ice-bath. Acrolyl chloride (2.1 mL, 26.5 mmol) was added and the solution was maintained at 0° C. for 20 minutes. Thereafter, the solution was allowed to warm up to room temperature and stirred at room temperature for 1 hour, at which point TLC indicated that all the alcohol compound has disappeared. The solution was poured into brine water and extracted by ether. The ether layer was dried over anhydrous magnesium sulfate. After removing the magnesium sulfate, the solvent was removed and the residue acrylate compound was purified by silica gel chromatography (developing solvent: hexane/acetone=1/1). The compound yield was 5.38 g (76%), and the compound purity was 99% (by GC).

c) Synthesis of Non-linear-optical Chromophore 7-FDCST

The non-linear-optical precursor 7-FDCST (7 member ring dicyanostyrene, 4-homopiperidino-2-fluorobenzylidene malononitrile) was synthesized according to the following two-step synthesis scheme:

A mixture of 2,4-difluorobenzaldehyde (25 g, 176 mmol), homopiperidine (17.4 g, 176 mmol), lithium carbonate (65 g, 880 mmol), and DMSO (625 mL) was stirred at 50° C. for 16 hr. Water (50 mL) was then added to the reaction mixture. The products were extracted with ether (100 mL). After removal of ether, the crude products were purified by silica gel column chromatography using hexanes-ethyl acetate (9:1) as eluent and crude intermediate was obtained (22.6 g). 4-(Dimethylamino)pyridine (230 mg) was added to a solution of the 4-homopiperidino-2-fluorobenzaldehyde (22.6 g, 102 mmol) and malononitrile (10.1 g, 153 mmol) in methanol (323 mL). The reaction mixture was kept at room temperature and the product was collected by filtration and purified by recrystallization from ethanol (yield 18.1 g, 38%).

d) Polymer Materials

Other materials besides the above monomers and initiator were purchased from Aldrich Chemicals, Milwaukee, Mich.

Production Example 1 Preparation of Homo-polymer by Azo Initiator Polymerization of Charge Transport Homopolymer (TPD Acrylate Type)

The charge transport monomer N-[(meth)acroyloxypropylphenyl]-N,N′,N′-triphenyl (1,1′-biphenyl)-4,4′-diamine (TPD acrylate) (2.5 g, 4.1 mmol,) was put into a three-necked flask. After toluene (9.8 g) was added and purged by argon gas for 1 hour, azoisobutylnitrile (9.4 mg) was added into this solution. Then, the solution was heated to 65° C., while continuing to purge with argon gas.

After 18 hrs of polymerization, the polymer solution was diluted with toluene. The polymer was precipitated from the solution and added to methanol, and the resulting polymer precipitate was collected and washed in diethyl ether and methanol. The white polymer powder was collected and dried. The yield of polymer was about 100%.

The weight average and number average molecular weights were measured by gel permeation chromatography, using a polystyrene standard. The results were Mn=8,344, Mw=12,600, giving a polydispersity of 1.51.

Production Example 2 Preparation of Copolymer by Azo Initiator Polymerization of Charge Transport Monomer and Non-Linear-Optical Precursor Monomer (TPD Acrylate/Chromophore Type 10:1)

The charge transport monomer N-[(meth)acroyloxypropylphenyl]-N,N′,N′-triphenyl (1,1′-biphenyl)-4,4′-diamine (TPD acrylate) (2.5 g, 4.1 mmol) and the non-linear-optical precursor monomer 5-[N-ethyl-N-4-formylphenyl]amino-pentyl acrylate (0.25 g), prepared as described in Production Example 1 were put into a three-necked flask. After toluene (9.8 g) was added and purged by argon gas for 1 hour, azoisobutylnitrile (9.4 mg) was added into this solution. Then, the solution was heated to 65° C., while continuing to purge with argon gas.

After 18 hrs polymerization, the polymer solution was diluted with toluene. The polymer was precipitated from the solution and added to methanol, then the resulting polymer precipitate was collected and washed in diethyl ether and methanol. The white polymer powder was collected and dried. The yield of polymer was essentially 100%. The weight average and number average molecular weights were measured by gel permeation chromatography as discussed above, using a polystyrene standard. The results were Mn=17,462, Mw=34,044, giving a polydispersity of 1.95.

To form the polymer with non-linear-optical capability, the precipitated precursor polymer (2.5 g) was dissolved in chloroform (12 mL). Into this solution, dicyanomalonate (1.0 g, 15.1 mmol) and dimethylaminopyridine (40 mg, 0.33 mmol) were added to the precursor polymer solution, and the reaction was allowed to proceed overnight at 40° C. The polymer was recovered from the solution by filtration of impurities as described previously, followed by precipitation into methanol, washing and drying.

Production Example 3 Preparation of Copolymer by Azo Initiator Polymerization of Charge Transport Monomer and Non-Linear-Optical Precursor Monomer (TPD Acrylate Chromophore Type 4:1)

The charge transport monomer N-[(meth)acroyloxypropylphenyl]-N,N′,N′-triphenyl (1,1′-biphenyl)-4,4′-diamine (TPD acrylate) (2.5 g, 4.1 mmol) and the non-linear-optical precursor monomer 5-[N-ethyl-N-4-formylphenyl]amino-pentyl acrylate (0.83 g) were obtained as described in Production Example 1 and were put into a three-necked flask. After toluene (9.8 g) was added and purged by argon gas for 1 hour, azoisobutylnitrile (9.4 mg) was added into this solution. Then, the solution was heated to 65° C., while continuing to purge with argon gas.

After 18 hrs of polymerization, the polymer solution was diluted with toluene. The polymer was precipitated from the solution and added to methanol, and then the resulting polymer precipitate was collected and washed in diethyl ether and methanol. The white polymer powder was collected and dried. The yield of polymer was about 100%. The weight average and number average molecular weights were measured by gel permeation chromatography, using a polystyrene standard. The results were Mn=17,462, Mw=34,044, giving a polydispersity of 1.95.

To form the polymer with non-linear-optical capability, the precipitated precursor polymer (2.5 g) was dissolved with chloroform (12 mL). Into this solution, dicyanomalonate (1.0 g, 15.1 mmol) and dimethylaminopyridine (40 mg, 0.33 mmol) were added, and the reaction was allowed to proceed overnight at 40° C. As before, the polymer was recovered from the solution by filtration of impurities, followed by precipitation into methanol, washing and drying.

Example 1 Preparation of Photorefractive Composition

A photorefractive composition testing sample was prepared. The components of the composition were as follows:

(i) TPD charge transport (described in Production Example 1): 60.0 wt % (ii) Prepared chromophore of 7FDCST 29.0 wt % (iii) Prepared TPA Acetate plasticizer 11.0 wt %

To prepare the composition, the components listed above were dissolved with toluene and stirred overnight at room temperature. After removing the solvent by rotary evaporator and vacuum pump, the residue was scratched and gathered.

To make testing samples, this powdery residue mixture was put on a slide glass and melted at 125° C. to make a 200-300 μm thickness film, or pre-cake. Small portions of this pre-cake were taken off and sandwiched between indium tin oxide (ITO) coated glass plates separated by a 105 μm spacer to form the individual samples.

Measurement Method and Geometry:

The diffraction efficiency of the material was measured through four-wave mixing (FWM) experiments performed in a tilted-sample geometry. The writing beams were incident on the sample with an inter-beam angle of 20° in air and the sample surface was tilted 60° relative to the writing beam bisector resulting in a grating period of 2.7 μm.

In continuous wave (CW) laser studies, two s-polarized beams (532 nm) of equal illumination flux (0.75 W/cm² each) were used to record the grating and a weak counter-propagating p-polarized beam (532 nm or 633 nm) probed the efficiency of the grating. In pulsed laser studies, single-shot of 3 mJ/cm² pulse energy (based on 1/e diameter) with 1 ns pulse-width was used. The steady-state diffraction efficiency was monitored through increasing bias field. The diffraction efficiency of the composite is calculated by dividing the diffracted signal over the incident signal power. In 633 nm reading case, the reading angle is deviated from 532 nm reading case by about 4 degrees.

Transient diffraction efficiency of the sample was done in the same geometry described above. Instead of a steady increase of field, a fixed (65 V/μm) bias field is applied and the recording beams (with a total irradiance of 1.5 W/cm²) were turned on through a shutter. The rise of diffraction efficiency is recorded in an oscilloscope. The rise curve is fitted to a bi-exponential growth function. The first, fast time constant is usually reported.

In pulsed laser studies, single shot pulses were used as recording beams. Each single pulse created its own grating. The single-pulse incident upon the sample had energy of 3 mJ/cm² with a pulsewidth of 1 ns. A bias voltage of 9 kV was applied. The fast rise and decay of hologram recorded by each single-pulse is monitored through an oscilloscope.

Phase Stability

The tested samples were put into an oven at 60° C. At certain intervals, the opaqueness of samples was checked by microscope. If there is no opaqueness and crystal inside the composition, the samples could be said to have good phase stability.

Linear Absorption Measurements

Linear absorption measurements were performed to determine the absorption coefficient of the photorefractive device. In this experiment, 532 nm laser beam was incident through the PR device vertically, the beam intensity before and after the PR device was monitored and the linear transmittance of the sample is given by: T=I _(Transmitted) /I _(incident)

The transmittance for 100 μm device is 81%, which is good enough to show reasonable grating forming ability at 532 nm.

Obtained Performance:

Initial diffraction efficiency (%): 60% at 60 V/μm Response time: 20 (ms) at 60 V/μm Phase stability (at 60° C.): good

Example 2 Preparation of Photorefractive Composition

A photorefractive composition testing sample was prepared. The components of the composition were as follows:

(i) TPD charge transport (described in Production Example 2): 49.5 wt % (ii) Prepared chromophore of 7FDCST 30.0 wt % (iii) Ethyl Carbazole 20.0 wt % Obtained Performance:

Initial diffraction efficiency (%): 74% at 60 V/μm Response time: 55 (ms) at 70 V/μm Phase stability (at 60° C.): good

Example 3 (Pulse Laser Experiment) Preparation of Photorefractive Composition

A photorefractive composition testing sample was prepared. The components of the composition were as follows:

(i) TPD charge transport (described in Production Example 1): 54.8 wt % (ii) Prepared chromophore of 7FDCST 20.0 wt % (iii) Ethyl Carbazole 25.0 wt % (iv) C60  0.2 wt % Obtained performance:

Initial diffraction efficiency (%): 36% at 60 V/μm Response time: <1 (ms) at 70 V/μm Phase stability (at 60° C.): good

Comparative Example 1

A photorefractive composition was obtained in the same manner as in the Example 1 except composition rate and components. The components of the composition were as follows:

(i) TPD homo polymer: 60 wt % (ii) Prepared chromophore of 7DCST 12.1 wt % (iii) Prepared chromophore of 7F-DCST 12.1 wt % (iv) Prepared chromophore of APDC 4.9 wt % (v) Prepared TPA Acetate plasticizer 13.3 wt % (vi) Purchased C60 sensitizer (MER, Tucson, AZ) 0.5 wt % Obtained Performance:

Initial diffraction efficiency (%): <1% at 70 V/μm (too big absorbance) Response time: not measurable Phase stability (at 60° C.): good

As shown in this comparative data, no grating formation ability was observed due to the 532 nm laser beam being too dark, although good diffraction efficiency was observed by 633 nm red laser beams.

Although the invention has been described with reference to embodiments and examples, it should be appreciated by those skilled in the art that various omissions, additions and modifications may be made to the compositions and methods described above without departing from the scope of the invention, and all such modifications and changes are intended to fall within the scope of the invention. Accordingly, the invention is only limited by the following claims. 

1. A method for modulating light, comprising the steps of: providing a photorefractive composition consisting essentially of a chromophore and a polymer, wherein the polymer comprises a repeating unit that includes a moiety selected from the group consisting of the following structures:

wherein each Q in (M-2) and (M-3) independently represents an unsubstituted or substituted alkylene group; wherein Rb₁-Rb₂₇ and Rc₁-Rc₁₄ in (M-2) and (M-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl; wherein the chromophore and the polymer are together selected to render the composition photorefractive upon irradiation by a green laser; wherein the chromophore is 4-homopiperidino-2-fluorobenzylidene malononitrile; and irradiating the photorefractive composition with a green laser to thereby modulate a photorefractive property of the polymer.
 2. The method of claim 1, wherein the polymer comprises a second repeating unit that includes a second moiety represented by the following formula:

wherein Q in (M-4) represents an unsubstituted or substituted alkylene group; R₁ in (M-4) is selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl; G in (M-4) is a π-conjugated group; and Eacpt in (M-4) is an electron acceptor group.
 3. The method of claim 2, wherein “G=” in formula (M-4) is selected from the group consisting of the following structures:

wherein Rd₁-Rd₄ and R₂ in (G-1) and (G-2) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl.
 4. The method of claim 2, wherein Eacpt in formula (M-4) is represented by a structure selected from the group consisting of the following structures:

wherein R₅, R₆, R₇ and R₈ in (E-2), (E-3), and (E-5) are each independently selected from the group consisting of hydrogen , C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl.
 5. The method of claim 1, wherein the polymer comprises a repeating unit selected from the group consisting of the following structures:

wherein each Q in (U-2) and (U-3) independently represents an unsubstituted or substituted alkylene group; and wherein Rb₁-Rb₂₇ and Rc₁-Rc₁₄ in (U-2) and (U-3) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl.
 6. The method of claim 5, wherein the polymer comprises a second repeating unit represented by the following formula:

wherein R₁ in (U-4) is selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl; G in (U-4) is a π-conjugated group; and Eacpt in (U-4) is an electron acceptor group.
 7. The method of claim 6, wherein “G=” in formula (U-4) is represented by a structure selected from the group consisting of the following formulae:

wherein Rd₁-Rd₄ and R₂ in (G-3) and (G-4) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl.
 8. The method of claim 6, wherein Eacpt in formula (U-4) is represented by a structure selected from the group consisting of the following formulae:

wherein R₅, R₆, R₇ and R₈ in (E-7), (E-8), and (E-10) are each independently selected from the group consisting of hydrogen, C₁-C₁₀ alkyl, C₄-C₁₀ aryl and C₄-C₁₀ heteroaryl.
 9. The method of claim 1, wherein the photorefractive composition further comprises a plasticizer and/or a sensitizer.
 10. The method of claim 1, wherein the photorefractive composition has an over 30% transmittance at a thickness of 100 μm.
 11. The method of claim 1, wherein the green laser is configured to irradiate the composition at a wavelength of about 532 nm.
 12. The method of claim 1, wherein the green laser is a continuous wave laser or a pulse laser. 